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Humans have brain sizes ranging up to 2000 cm³, with the average being about 1400 cm³. Larger brains correlate with higher intelligence, particularly between different species. This means that larger brains tend to be more intelligent, though this is often not the case, between species or within species. The sheer size of the brain is relevant for two reasons. Most obviously, a small brain simply cannot hold as many brain cells as a large one. Less obvious, but more important, is that the true quality of a brain must be measured by the complexity of linkages between cells.

Modern studies using MRI imaging shows a weak to moderate correlation between brain size and IQ (Harvey, Persaud, Ron, Baker, & Murray, 1994) and have shown that brain size correlates with IQ by a factor of approximately .40 among adults (McDaniel, 2005). In 1991, Willerman et al. used data from 40 White American university students and reported a correlation coefficient of .35. Other studies done on samples of Caucasians show similar results, with Andreasen et al (1993) determining a correlation of .38[1], while Raz et al (1993) obtained a figure of .43 and Wickett et al. (1994) obtained a figure of .40. The correlation between brain size and IQ seems to hold for comparisons between and within families (Gignac et al. 2003; Jensen 1994; Jensen & Johnson 1994). However, one study found no within family correlation (Schoenemann et al. 2000).

The brain is a metabolically expensive organ, and consumes about 25% of the body's metabolic energy. Because of this fact, although larger brains are associated with higher intelligence, smaller brains might be advantageous from an evolutionary point of view if they are equal in intelligence to larger brains. Skull size correlates with brain size, but is not necessarily indicative.

Brain size is a rudimentary indicator of the intelligence of a brain, and many other factors affect the intelligence of a brain. Higher ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks.

Here is a list of some species, along with their rough average brain sizes:

A study on twins (Thompson et al. 2001) showed that frontal gray matter volume was correlated with g, a measure of generalized intelligence, and is highly heritable. A related study has reported that the correlation between brain size (reported to have a heritability of 0.85) and g is 0.4, and that correlation is mediated entirely by genetic factors (Posthuma et al 2002). Note that none of the MRI studies have studied racial differences.

Other neurological paramaters have been associated with IQ. Haier et al. (1995) found a correlation of -.58 between glucose metabolic rate (an indicator of energy use) and IQ. This suggested that intelligence is associated with more efficient brains. Others found a positive correlation between IQ and GMR (DeLeon et al 1983; Chase et al 1984). It seems like difference in results comes from different cognitive tasks (complicated vs. simple) that were performed by examinees (Fidelman , 1993).